Phase Transition And Magnetismof Mn-based All-d-metal Heusler Alloy

In this thesis,we focused on the properties of all-d-metal Heusler alloys.Combined with the first-principles calculation and experiment,the crystal structure,martensitic transformation and magnetic structural transformation in some all-d-metal Heusler alloys were investigated theoretically and experimentally.The structure,magnetic properties and martensitic transformation of all-d-metal Heusler alloys Zn₂YMn(Y=Fe,Co,Ni,Cu)were predicted based on first-principles calculations.We have found that Zn₂YMn prefer to form a L2₁structure comparing with XA structure.The total magnetic moment of Zn₂YMn are all larger than 3?_B/f.u.,mainly due to the contribution of the partial moment of Mn atom.The total energy of the tetragonal martensitic phase is lower than that of the cubic austenite phase in Zn₂YMn(Y=Fe,Co,Ni),indicating that martensite transformation may occur in this series of materials.For example,the energy difference between austenite and martensite calculated in Zn₂Fe Mn is as high as 0.31 e V/f.u.At the same time,large c/a values were observed in martensitic type Zn₂Fe Mn and Zn₂Co Mn,indicating that they may have large phase transformation strains.The effects of Zn substitution for Ga on the phase transition and magnetic properties of Heusler alloy Ni₂Mn Ga_1-xZn_x(x=0,0.25,0.50,0.75 and 1.00)were investigated.When Zn replaces Ga in Ni₂Mn Ga,Zn enters the Ga(D)position,similar to the behavior of the main group element and the equilibrium lattice constant decreases with increasing Zn content.However,the influence of Zn doping on its magnetic properties is not obvious.The spin moment of Mn contributes the most to the total magnetic moment and is parallel to the magnetic moment of Ni.When x=0.5,the calculated energy difference?E_Mbetween austenite and martensite is the largest,indicating that the partly substitution of Zn for Ga can increase the martensitic transformation temperature due to the influences of cell volume effect and valence electron concentration e/a.Zn-doped Ni₂Mn Ga and Mn₂Ni Ga have been synthesized successfully.It is found that Zn doping does not introduce secondary phase in the two materials,and the increase of Zn content can significantly increase the phase transition temperature of the them.Thus Zn doping can be used as a possible way to adjust the phase transition temperature of Ni-Mn-Ga and other Heusler type MSMAs.This is of great interest for reducing the production cost of MSMAs and promoting the industrial application of magnetic shape memory alloys.Mn-Ni-Co-Ti alloy is a newly reported all-d-metal Heusler alloy.We studied the electronic structure and magnetic structural transformation of Mn₂Ni_1.5Ti_0.5and Mn₂Ni_1.25Co_0.25Ti_0.5theoretically.In these alloys,when extra Ni/Co substitute for Ti,the same amount of Mn atoms will be driven into the D position of the cubic lattice.The spin magnetic moment of Mn(D)is parallel to the Mn moment at the B site in the austenite phase,but becomes antiparallel in the martensite phase.This explains the phenomenon of the decrease in saturated magnetization observed during the phase transition.Compared with Mn₂Ni_1.5Ti_0.5,the doping of Co in Mn₂Ni_1.25Co_0.25Ti_0.5stabilizes the parallel arrangement of Mn(B)and Mn(D)spin moments in the austenite phase.The calculation also found that the volume shrinkage before and after the Mn₂Ni_1.25Co_0.25Ti_0.5phase change was as high as 3.5%.